1. The Field of the Invention
This invention relates to systems and methods for geophysical exploration and, more particularly to a novel system and method for conducting geophysical exploration using a real time data collection and processing survey system and signals from a global positioning satellite network.
2. The Prior Art
Each year, billions of dollars are spent on various types of geophysical surveys. The majority of these surveys are presently conducted by oil companies in search of petroleum and related fuels. In addition, however, many geophysical surveys are conducted each year which relate to mineral, geothermal, and ground water exploration as well as oceanography, engineering, and geophysical research in general. As a result, during recent years, the geophysical exploration industry has grown steadily, and this growth pattern is expected to continue for several years to come.
One of the most widely used geophysical survey techniques is seismic surveying, which may be conducted either on land or at sea. Some recent studies indicate that expenses relating to seismic surveying have accounted for approximately 75% of all expenditures for geophysical exploration during recent years.
Despite the frequent use of seismic surveying, however, it has a number of significant drawbacks which reduce both its effectiveness and its desirability in geophysical exploration. First, seismic survey techniques are extremely costly. For example, a single line mile of surveying on land using seismic survey techniques may cost several thousand dollars. While marine seismic surveys may be somewhat less expensive, marine surveys may also cost over $1,000 per line mile. In addition to the high cost of seismic surveys, seismic surveys are known to be less effective in defining subsurface structure and lithology in many frontier areas in which geophysical exploration may be desirable. For example, regions in the Pacific Northwest covered by basalt and other volcanic formations and the Intermountain Overthrust Belt are very difficult to effectively explore using seismic techniques.
In addition to the problems noted above, the drilling costs which are required in order to confirm a geophysical find have steadily increased. Moreover, geophysical exploration has surged, and competition in the geophysical exploration industry is increasing rapidly. As a result of these several factors, many attempts have been and are currently being made to develop other systems and methods for geophysical exploration. At the present time, one of the most promising of such methods appears to be that of airborne geophysical surveying, the use of which has risen steadily over the past several years. In particular, due to the significant refinements in aeromagnetic instruments, aeromagnetic surveys appear quite promising as an alternate geophysical exploration technique.
Aeromagnetic surveys are based upon the measurement of the earth's magnetic field over a particular region. It is generally known that the earth has a variable, magnetic field which is influenced by the presence of subsurface igneous and metamorphic rocks and sediments which contain magnetic particles, such as, for example, mixed oxides of iron and titanium and other magnetic ore bodies. The magnitude of the influence of such subsurface structures on the natural magnetic field in a given area is referred to as a magnetic anomaly, and such an anomaly can be measured quite precisely using an instrument called a magnetometer.
Advances in the design and sensitivity of magnetometers led to the first practical airborne use of the instrument in 1944. Since that time, further advances in instrumentation and data interpretation have led to broader application and sharply increasing use of aeromagnetic techniques for petroleum, mineral and geothermal exploration. Using such techniques, anomalous high or low values of magnetic field strength are of particular interest and are plotted as contours on appropriate maps. Properly processed and interpreted data can then be used to target the possible location and size of potential ore bodies, geothermal reservoirs, hydrocarbon traps or cultural artifacts. Thereafter, suspected finds are confirmed by the use of ground survey techniques and drilling.
Aeromagnetic surveys are presently conducted using a suitable aircraft which is equipped with a magnetometer, recording equipment, a sensitive altimeter, and a camera. The aircraft is manned by a flight crew which may consist of from one to three individuals, depending upon the particular requirements of the survey. For example, if the survey is being flown at a quite high altitude above ground level, the pilot may perform the aeromagnetic survey alone. However, when higher resolution aeromagnetics are desired, thus requiring the survey to be flown closer to the ground, the pilot is typically very busy flying and a separate navigator is usually required. In some cases, it may also be desirable to have a separate geophysical technician on board the aircraft during the survey.
Before an aeromagnetic survey is conducted, the survey is first planned by the contractor. After it is determined where the survey is to be conducted, the elevation at which the data is to be collected, and the spacing of the flight lines, the contractor maps out the survey by drawing the selected flight lines on aerial photographs of the survey area or by drawing such flight lines on topographical maps of the survey area.
Once the survey is thus planned, the flight crew is mobilized to the survey area and the survey is commenced. By following the flight lines which were previously drawn on the aerial photographs or topographical maps of the survey area, the navigator directs the pilot along the appropriate flight lines across the survey area. During the flight, the mangetometer is measuring the magnetic field, and such meansurement is being recorded by the recording equipment. At the same time, the camera takes photographs of the ground over which the plane is flying.
After the survey has been conducted and the data recorded, the flight crew returns to its base. The data can then begin to be analyzed and interpreted. In interpreting the data, a flight line positioner first views the film frame by frame and matches the photos taken by the camera on the aircraft with a large aerial photograph of the survey area. In this way, the flight line positioner determines the plane's actual position during each flight segment of the survey. Then, once the actual flight lines have been determined by the flight line positioner, the data which was recorded during the flight are corrected for diurnal changes in the earth's magnetic field and are plotted at the appropriate points along the flight lines. Finally, the data are properly contoured and are thereafter ready to be analyzed.
It will be readily appreciated that the prior art aerial survey technique described above may give rise to a number of problems and difficulties. First, it may be quite difficult to accurately position the aircraft during the survey using the topographical maps or aerial photographs on which the flight lines have been drawn. This is particularly true in areas of low relief over which the aircraft may need to travel. Positioning may also be a problem over heavily forested, totally denuded, snowcovered, or water-filled areas. Similarly, it may be extremely difficult for the flight line position to later match the photographs taken by the aircraft's camera over such areas with the aerial photograph of the survey area.
In addition, since aeromagnetic surveys are typically flown quite close to the ground, the camera image of the survey area may be quite blurred, with a very small angle of acceptance. Consequently, only a very small visual sample of the ground may be obtained on the film, which in turn also makes flight line recovery very difficult.
An additional difficulty inherent in prior art aeromagnetic techniques arises from the fact that the equipment which the aircraft must carry, together with the required number of crew members, may be quite heavy. Accordingly, a large plane or helicopter is typically required in order to carry all of the equipment and crew members. Unfortunately, however, large planes have a relatively high stall speed. For example, a large fixed wing plane may have a stall speed of over 100 miles per hour. Consequently, when using such a plane, the survey must be flow at over 100 miles per hour in order to avoid stalling. It will be appreciated, however, that flying an aircraft relatively close to the ground at such a speed may be quite dangerous; and it may, therefore, be difficult to find pilots willing to fly the planes in this manner on a regular basis. If, on the other hand, a helicopter is chosen for purpose of conducting a survey, the cost of the survey increases substantially. It is not uncommon at the present time, for example, for helicopters to cost more than $1000 per hour to rent.
In addition to the above, one very significant drawback associated with prior art aeromagnetic survey techniques is that a substantial amount of time is required before a finished survey product is available for use. First, it may take two to three weeks just to complete the task of flight line recovery for the survey. Then, the data must be corrected, plotted, and contoured. Thus, even assuming that all of the instruments on the aircraft were functioning properly during the survey, it may be a month or more before the data acquired during the survey are actually ready for use and analysis. Even more troublesome is the fact that malfunctioning equipment and/or faulty data may not be discovered until weeks after the survey has been flown, and this may require that the survey be reflow at a later date.
Accordingly, it would be an improvement in the art to provide a system and method for conducting geophysical surveys in which the survey pattern may be accurately controlled while collecting data. It would also be an improvement in the art to provide a geophysical survey system in which the area collection equipment is lightweight and the crew requirements are minimal, thereby permitting the use of a small aircraft for aerial survey applications. In addition, it would be an improvement in the art to provide a system and method for conducting geophysical surveys in which the data may be processed and analyzed in real time while the survey is underway. Further, it would be an improvement in the art to provide a system and method for conducting geophysical surveys in which both the integrity of the data and the accuracy of the instruments may be verified before the data acquisition vehicle (DAV) leaves the survey area. Such a system and method are disclosed and claimed herein.